I've lived all over the USA and speed limits vary a lot as does the behavior of drivers.

Where I live now all the speed limits are 10-20 mph slower than down in Texas for example. Modem cars handle high speeds very well. During the summer to here you could safely drive these roads much faster than the posted speed limit. But it also snows up here. During the winter it would often actually be quite reckless to drive the posted speed limit because of the slippery road conditions.